1. Field of the Invention
This invention relates to the field of using remote radar systems for mapping terrain and structures found on the terrain in three dimensions.
2. Description of the Related Art
Synthetic Aperture Radar (SAR) was invented as a military reconnaissance instrument in the 1950's and further developed through the 1960's as a way to image ground terrain at high resolution, typically from an aircraft. It had advantages over passive optical reconnaissance systems since it could operate at night and see through clouds. Furthermore, due to the nature of SAR imaging techniques, the sensor could operate at very large standoff distances without sacrificing resolution. During the early 1970's interferometric SAR systems were invented. In such systems, the phase difference of the radar echoes received at two antennas spaced perpendicular to the aircraft's velocity vector are used to estimate the elevation angle from the SAR to the terrain (and thus the height of the terrain) (Richman, Graham). Using interferometric SAR (InSAR), it was possible to produce terrain maps as well as high-resolution imagery that could be displayed on the terrain map.
Throughout the 1980's and 1990's such InSAR instruments and processing techniques were developed for scientific, military and commercial applications (Goldstein et al. U.S. Pat. No. 4,551,724, Brown U.S. Pat. No. 5,170,171). Madsen et al. present an end-to-end solution for processing 2-antenna InSAR data for elevation extraction in U.S. Pat. No. 5,659,318. Encouraged by these maturing radar and processor technologies, in February 2000 the US Government flew an InSAR instrument on Space Shuttle Endeavour. During the 11-day mission, the instrument successfully mapped the terrain of over 80% of the Earth's landmass, and almost 100% of the landmass located between 60 degrees north latitude and 54 degrees south latitude.
Interferometric SAR systems require two receive antennas that view a common area of terrain from a slightly different elevation angle, where the elevation angle is the angle between the nadir vector (pointing from the SAR platform straight down to the earth's surface), and the radar line-of-site vector. The two observations may be made at the same time from different locations on the same platform, or at different times from possibly different platforms. There are various constraints on the radar hardware and InSAR geometry that are well known in the art; but of particular importance is the separation, or baseline, between the two antennas. This separation leads to the slightly differing elevation angles viewed by each antenna. The larger the separation is, the more sensitive the InSAR system is to terrain height variation. The separation therefore must be large enough to produce a measurement of radar phase differences that may be usefully converted into terrain heights, but not so large as to become incoherent making phase measurements very noisy and accurate elevation determination impossible. For these reasons, design of a two-antenna InSAR system requires a trade-off between height sensitivity and interferometric phase noise.
A related and limiting issue involves what are known as height ambiguities. Since the phase measurement of any interferometer only allows the measurement of the modulus of the total phase difference (that is, all the phase measurement are between −π and π radians), there are multiple heights that can yield the same measured phase. Such phase measurements are termed wrapped. However, it is the absolute phase measurement (not wrapped) that is required for determining absolute height of a pixel. Given, or assuming, a true height of some (control) point in the image, an unwrapped phase for that point may be determined Wrapped phases for the nearby points may then be unwrapped by assuming heights do not differ from the control point by an amount that would cause the phase difference to change by +/−π/2 radians (Ghiglia and Pritt) through adjacent pixels. This height is called the ambiguity height. The unwrapping process may be continued throughout the entire two-dimensional SAR scene, and will result in an accurate terrain map of the area relative to the control point height, assuming there are no height discontinuities greater than half of the ambiguity height within the scene. In the case where there are such discontinuities, InSAR elevation estimates will be in error by some multiple of the height ambiguity, and this error will propagate throughout the scene due to the unwrapping process. Some techniques for identifying ambiguity errors have been developed, but no methods for their robust repairs are possible (Ghiglia and Pritt). Current best practice is to “null” out any known problematic points, i.e. not include height measurements for those points. This produces an elevation map with nulls, or holes.
Various unwrapping approaches have been developed (e.g. Least-Squares approach as described by Ghiglia et al. in U.S. Pat. No. 5,424,743), that mitigate these discontinuity errors and eliminate nulls, but essentially have the effect of reducing the accuracy of terrain models that contain height ambiguities by spreading their deleterious effect throughout the scene.
Height ambiguities become a limiting factor for high-resolution InSAR systems. The resolution of modern SAR systems is such that having the capability to resolve human-scale objects in two dimensions is common To be useful, any three-dimensional mapping system based on such high-resolution 2D SAR data requires a commensurate height resolution. In order to obtain high precision in the measurement of height, a traditional 2-antenna system would require a large baseline. However, as described above, this necessarily introduces a small height ambiguity and this limits the amount of terrain slope that can be correctly measured over small distances, and hence the ultimate utility of the 3D imager. An approach for addressing this height ambiguity challenge was presented by Jakowatz [Jakowatz, 1996] through the simultaneous use of a small and a large baseline interferometer.
Another factor that limits the utility of the 2-pass/antenna InSAR technique involves pixels with multiple scatterers at different heights. Such an occurrence is likely when pixels are in layover, which happens frequently when imaging discontinuous terrain such as an urban landscape. Each scatterer in the pixel will contribute coherently to the SAR image pixel's response, and thus the phase of the pixel, and derived height of that pixel, will be based on the mixed phase value. The derived pixel elevation will correspond to some height between the actual heights of the scatterers, depending on the relative radar cross-section of the scatterers.
For these reasons, the generation of high-resolution and precision maps of areas containing discontinuous heights, such as built-up urban areas, is not practical with two-antenna InSAR systems.